Network analyzers are devices that are used to determine the radio frequency (RF) characteristics of various devices under test (DUTs). Network analyzers typically operate by sweeping a stimulus signal across a range of frequencies and applying the stimulus signal to a DUT and measuring the response of the DUT. The measurements generated by network analyzers typically possess inaccuracies due to a number of issues. For example, impedance mismatches associated with the network analyzer and/or a test set-up create errors in the measurements.
Calibration techniques can be applied to enable post-processing of the measurement data to mitigate the errors in the measurement data. In general, network analyzer calibration occurs by applying a stimulus signal to “standards” (devices having known or estimated characteristics) and estimating the systematic errors from the measurements. The “SOLT” type calibration methodology occurs by employing a “short” standard, an “open” standard, a “load” standard, and a “thru” standard. A methodology (the “unknown” thru method) has been developed that enables the calibration of a network analyzer using a thru standard without requiring the characteristics of the thru standard to be determined. The “unknown thru” methodology relies upon the general theory of eight term network analyzer error correction.
Frequency translation devices (FTDs), such as mixers, modulators, demodulators, and the like, present difficulties to the operation and calibration of network analyzers. Specifically, when a network analyzer makes a particular measurement, the measurement is taken at the same frequency as the current frequency of the stimulus signal. Since the output frequency of an FTD is different than the input to the FTD, ordinary network analyzer measurements cannot be used. To address this problem, a test methodology that employs three mixers and three sets of measurements has been developed that enables measurement of the characteristics of FTDs using a network analyzer.
Another methodology enables a network analyzer to be calibrated to enable vector error correction to be applied to FTD measurements. The methodology involves using a “reference” FTD that has been previously characterized to serve as the thru device. However, FTDs are subject to drift. To obtain the greatest amount of accuracy, the reference FTD must be recharacterized every time a calibration is to be performed. Specifically, failure to recharacterize causes an error in the calibration that is equal to the drift of the reference FTD. Also, the two measurement tiers (characterizing the reference FTD and measuring other FTDs of interest) creates a greater amount of measurement complexity and uncertainty.